To drive all the electronic elements within a thin film transistor liquid crystal display (TFT-LCD), the TFT-LCD requires a level shifter to shift from an input voltage level, which may come from a computer-like device, to an output voltage level. Because the threshold voltage of TFTs is larger than that of CMOSs, most TFT-LCDs of prior art utilize a detection circuit to avoid bad performance of the level shifter caused by excessive-low input voltage.
FIG. 1 shows a level shifter of prior art. With reference to FIG. 1, the basic structure of the level shifter includes a shift circuit 32, a first detection circuit 27, and a second detection circuit 29. The shift circuit 32 includes a pair of input transistors, a first transistor 2 and a second transistor 4, and a pair of load transistors, a third transistor 6 and a fourth transistor 8. The first detection circuit 27 includes a fifth transistor 10, a first input terminal 18, and a first current source 14. The second detection circuit 29 includes a sixth transistor 12, a second input terminal 20, and a second current source 16. The input signals of the first input terminal 18 and the second input terminal 20 need complementary voltage levels. The fifth transistor 10 and the sixth transistor 12 are configured herein to detect the threshold voltages of the first transistor 2 and the second transistor 4 respectively to maintain that the operations of the first transistor 2 and the second transistor 4 are in an appropriate mode. Accordingly, the level shifters can perform as expected.
FIG. 2 shows another level shifter of prior art. The basic structures illustrated in FIG. 1 and FIG. 2 are identical except for different gate connections of the third transistor 6 and the fourth transistor 8. FIG. 3 shows a further improved level shifter of prior art, which is identical to the level shifter illustrated in FIG. 1 except the addition of a seventh transistor 13. With the seventh transistor 13 and its specific connection way, the level shifter in FIG. 3 can, without the second input terminal 20, perform the functions of level shifters in FIG. 1 and FIG. 2.
The operation frequency of TFT-LCDs has been increasing due to ever-increasing amount of data that needs to be processed during its transmission. However, the always-active status of the first transistor 2 and the second transistor 4 limits the speed improvement of the aforementioned level shifters. Besides, the aforementioned level shifters have two DC routes which go from the current sources (the first current source 14 and the second current source 16) to ground via the detection transistors (the fifth transistor 10 and the sixth transistor 12). These two DC routes result in unnecessary power consumption while the power supply 30 is supplied regardless whether the level shifters are operating or not because the connections of the fifth transistor 10 and the sixth transistor 12 require this constant power supply.